The present invention relates to an electromagnetic induction coil sensor suitable for use in an apparatus for detecting rock movement and other sensing applications.
In U.S. patent application Ser. No. 09/069,478 filed Apr. 29, 1998, now U.S. Pat. No. 6,084,413, there are described short comings in available apparatus for detecting rock movement. Scientists have been trying to perfect techniques for collecting data regarding rock movement, such as that which is caused by the flow of water in a river or stream (riverbed movement), which is necessary for a thorough study of erosion, flooding, sedimentation and the like. One technique that scientists have employed involves collecting rocks from the site under investigation and implanting a small magnet inside each rock. The rocks are then placed strategically upstream of their anticipated path of travel. After an event of flooding, the new positions of the rocks are traced by detecting the magnet field of the magnets inside each rock. The movement which the rocks have undergone can then be deduced from the changes in the positions of the rocks. One disadvantage of this method is that it is costly to collect rocks, implant a magnet in each rock, strategically place the rocks back at the site under investigation and detect the rocks having magnetic implants after the event. Another disadvantage of this method is that, in adding magnets to the rocks and placing the rocks back into the environment, there is a human intervention that can skew the data received.
Another technique that scientists use based upon the fact that a proportion of rocks at any site under investigation have a naturally occurring remnant magnetization that is strong enough to be detected by shock coils. The signal level which can be obtained using existing technology for detecting the remnant magnetization in a rock that is capable of being detected is approximately 10 microvolts. By working out a statistical average of the percentage of rocks at a site under investigation that are capable of being detected, rough estimates can be made as to the real number of rocks being moved. This method of detection of rock movement requires a considerable amount of study to arrive at a statistical average as to the percentage of rocks at the site under investigation having a remnant magnetism of sufficient strength to generate a signal of magnitude of 10 millivolts. It also requires the use of expensive amplifiers to condition the signals.
The above mentioned patent application then goes on to describe an alternative method and apparatus for detecting rock movement. This new approach is based upon changes in magnetic permeability in medium proximate to the coil as a result of rock movement in accordance with Faraday""s law of electromagnetic induction.
Magnetic induction B is given by
B=xcexc.H
where xcexc is the magnetic permeability of a given medium and H is the applied magnetic field. The magnetic flux "PHgr" through one loop of an electrical coil is given by
"PHgr"=B.A. cos xcex8
where B is as defined above, A is the cross sectional area of the loop, and xcex8 is the angle between the vector of B and the direction perpendicular to the plane of the loop. As the magnetic flux "PHgr" changes with time t an electromotive force EMF is generated which is proportional to the product of the rate of change of the magnetic flux and the number of loops in the electrical coil N
EMF=xe2x88x92N.(d"PHgr"/dt).
With the alternative approach proposed, the rocks do not have to have any remnant magnetism, as what is being measured is magnetic susceptibility; the ability of the rock to be magnetized. This method of detecting rock movement based upon induced electromotive force due to the change of magnetic permeability has been found to be much more sensitive to rock movement than the previously described method that relied upon detection of rocks with remnant magnetism. The method detects changes in the magnetic permeability of the medium proximate to the sensor as an object passes close to the sensor. The method is capable of generating signals which are typically between 3 and 10 millivolts as a rock passes by the sensor; which is about 1000 times greater than the signals generated by detecting remnant magnetism.
A key component of the apparatus for detecting rock movement is an electromagnetic induction coil sensor. This electromagnetic induction coil sensor can be used in other applications, not related to rock detection. Such a sensor can be used to detect changes in magnetic permeability due to cracks, metal fatigue in a material or structure. It can be used to monitor traffic on highways or railways. It can be used to detect movement and speed of movement of conveyors. It can be used to detect movement as part of a security system. It can be used to detect non-uniform distributions of elements in a given alloy or mixture. It can be used to detect movements of moving parts in any machine.
During the prosecution of the parent application, the closest prior art disclosed in a search by the United States Patent and Trade Mark Office was U.S. Pat. No. 4,384,252 which issued to Kolter. The Kolter reference discloses an iron casing, a rod like permanent magnet which serves as a central magnetic core, and electrically conductive wire wound around a bobbin which encircles the permanent magnet.
The effectiveness of an electromagnetic induction coil sensor is dependent upon the strength and uniformity of the magnetic field at the central magnetic core. The Kolter reference is viewed as having inherent limitations upon the strength and uniformity of the magnetic field at the central magnetic core. With the electromagnetic induction coil of Kolter a permanent magnet is used at the central magnetic core. A permanent magnet has field lines that extend outwardly from the magnet like branches on a tree. This magnetic field of a permanent magnet is strongest at its poles and becomes progressively weaker until in the middle the positive and negative magnetic field cancel each other out. This results in the sensitivity of the sensor being reduced at the midpoint of the magnet.
What is required is a electromagnetic induction coil sensor that has a stronger and more uniform magnetic field.
According to the present invention there is provided an electromagnetic induction coil sensor which includes a casing with high magnetic permeability with a magnet disposed within the casing. The magnet has a central hole. A core with high magnetic permeability is disposed within the central hole of the magnet. The core is in physical contact with the casing. A multi-loop coil of conductive wire is disposed within the central hole of the magnet encircling the core. The multi-loop coil is insulated from the magnet.
The electromagnetic induction coil sensor, as described above, has a stronger and more uniform central magnetic core than Kolter""s. The magnetic field lines tend to become concentrated in the central hole of the magnet. There is a difference between the strength of the magnetic field in the middle as compared to ends. Instead of being weaker in the middle, the magnetic field is stronger in the middle. This increase in the magnetic field in the middle depends upon the hole diameter, but is roughly a two fold increase. In order to increase sensitivity of a electromagnetic induction coil sensor, one reduces the size of the core. Kolter has less flexibility when making such an adjustment, for he uses a permanent magnet as his core. When Kolter reduces the cross-sectional area of his core, he is also reducing the amount of material in his magnet. This serves to reduce magnetic field strength and can reduce, rather than increase, sensitivity.
Although beneficial results may be obtained through the use of the electromagnetic induction coil sensor, as described above, the electromagnetic induction coil sensor is often required in environments that require resistance to vibration or exposure to the elements. Even more beneficial results may, therefore, be obtained when the core, the multi-loop coil and the magnet are secured within the casing by adhesive. The adhesive prevents relative movement of the components within the casing. Where the electromagnetic induction coil sensor is going to be exposed to water, the adhesive should be water proof. Beneficial results have been obtained through the use of epoxy. The use of adhesive enables the multi-loop coil to be insulated from the magnet by a insulating gap.
Although beneficial results may be obtained through the use of the electromagnetic induction coil sensor, as described above, the pattern of field lines is a function of the distance between the core of the sensor and the outer edge of the casing. Even more beneficial results may, therefore, be obtained when the casing has an outwardly projecting lip. This outwardly projecting lip increases the distance to the outer edge of the casing and changes the height of the field lines.